In known methods for making integrated circuits of a class commonly known as "gate arrays", generic base layers are formed on a semiconductor wafer. The generic base layers usually define a large number of unconnected semiconductor devices such as bipolar transistors, MOS transistors or CMOS transistor pairs.
The semiconductor devices of the generic base layers are usually arranged on the wafer in an array of regularly spaced rectangular regions. Each rectangular region contains an identical arrangement of semiconductor devices and adjacent rectangular regions are separated by predefined empty bonding pad regions in which bonding pads will eventually be formed and predefined empty scribe regions in which the wafer will eventually be scribed to separate the rectangular regions to provide separate integrated circuits.
To complete the fabrication of integrated circuits implementing a specific electronic function, customized interconnection layers appropriate to that function are formed over the rectangular regions to interconnect the semiconductor devices so that they will perform the desired electronic function. The arrangements of interconnection layers applied to each of the rectangular regions are identical, and none of the interconnections cross the empty scribe regions which separate adjacent rectangular regions. The interconnection layers include bonding pads in the bonding pad regions which are often located around the periphery of the rectangular regions. The bonding pads are used for making external electrical connections to the integrated circuits. Once the interconnection layers are in place, the semiconductor wafer is scribed in the scribe regions, and the rectangular regions of the wafer are separated into individual die which are separately packaged as distinct integrated circuits.
The use of generic base layers in gate arrays simplifies, speeds up, standardizes and reduces the cost of designing integrated circuits to perform new electronic functions. Because the generic base layers are common to many integrated circuits and provide semiconductor devices which can be interconnected to perform many different functions, the design of integrated circuits performing new electronic functions requires only the design of a new set of interconnection layers to interconnect the devices of the generic base layers to perform the desired function. This is much simpler than designing a completely new set of base layers and interconnection layers as is done in the manufacture of custom integrated circuits. Moreover, because the electrical characteristics of devices formed in the generic layers are known to be good from previous uses of the generic base layers in other integrated circuits, less design verification effort is required.
The use of generic base layers also speeds up and reduces the cost of manufacturing integrated circuits based on those generic base layers. Because the generic base layers can be used in many different integrated circuits, they can be manufactured in large volumes to provide economies of scale. Wafers carrying only the generic base layers can be manufactured in large quantities and stockpiled for future completion by the addition of customized interconnection layers required for the implementation of new electronic functions.
In summary, the use of generic base layers permits manufacturers to meet many integrated circuit needs of their customers faster and cheaper than would be possible if each integrated circuit were designed without such generic layers as in the design of custom integrated circuits.
Unfortunately, there are limits to the benefits of the gate array design and manufacturing strategy which is described above. The complexity of the electronic functions that can be implemented using the generic base layers depends on the number and type of unconnected semiconductor devices provided in each of the regularly spaced rectangular regions of the wafer. If a very large number of unconnected semiconductor devices is provided in each rectangular region of the generic base layers to ensure that complex electronic functions can be implemented, then each of the rectangular regions will be relatively large, and a limited number of them will fit on a semiconductor wafer. This will limit the yield of integrated circuits per semiconductor wafer, a factor which will be reflected in the cost per integrated circuit. In this case, integrated circuits performing relatively simple electronic functions which do not require the large number of semiconductor devices provided in each rectangular region of the generic base layers will cost almost as much as integrated circuits performing significantly more complicated electronic functions implemented using the same generic base layers. However, if a smaller number of unconnected semiconductor devices is provided in each rectangular region of the generic base layers to increase the yield of integrated circuits per semiconductor wafer, it will not be possible to implement some more complex electronic functions on a single integrated circuit. Moreover, if different sets of generic base layers are provided for different levels of design complexity, this erodes the economies of scale which the use of generic base layers is meant to provide.